Measurement of the polarization vector of the e+ from the decay of polarized μ+ as a test of time reversal invariance

The complete polarization vector of the positrons from the decay of polarized muons has been measured for the first time with one experimental setup. The μPT experiment at the Paul Scherrer Institute determines the three polarization components simultaneously with the same apparatus by making use of three different effects (spatial and temporal dependence of annihilation-in-flight with polarized electrons as well as muon decay asymmetry). The use of a stroboscopic method greatly reduces systematic errors. The energy dependence of the transverse polarization component PT1, which lies in the plane spanned by muon spin and positron momentum, yields the low energy parameter η and thus an improved model-independent value of the Fermi coupling constant. A non-zero value of the transverse component PT2, which is perpendicular to the above-mentioned plane, would be the first observation of time reversal violation in a purely leptonic decay. The preliminary results are PT1 = (5 +/- 16) × 10−3, PT2 = (1 +/- 16) × 10−3 and PL = (1.09 +/- 0.15).